A liquid crystal display, or LCD, is a display apparatus that utilizes an image display panel formed of two transparent sheets of polarizing material separated by a liquid containing rod-shaped crystals where the polarizing axes of the two sheets are aligned perpendicular to each other. The LCD is constructed to display an image by passing an electric current through the liquid that causes the crystals to align to block light. Each crystal can be controlled individually and basically acts like a shutter. When the current is applied to specific pixel-like areas, those crystals align to create dark area, or images. The dark areas are combined with light areas to create text and images on the panel. LCD panels do not emit light but are usually back-lit or side-lit for better viewing of the text and images on the display panel. In general, back-lit LCDs are used for larger screens (generally considered larger than about 24 inches in the diagonal), whereas side-lit LCDs are used for smaller screens, usually in conjunction with optics for light distribution so the light does not appear to come from the side.
In a liquid crystal display, the back or side lighting used to illuminate and enhance the viewing of the image display panel generates heat and thus constitutes a source of heat, which causes the temperature of the liquid crystal display as a whole to rise. Traditionally, a single light source, or a plurality of heat generating light sources, such as fluorescent lights, such as cold cathode fluorescent lamps (CCFLs) or flat fluorescent lamps (FFLs), have been used as the illuminating light. Recently, arrays of light-emitting diodes, or LED's, are being used as the light source to eliminate environmental issues occasioned by fluorescent lamps and to improve the range of colors capable of being displayed.
The heat generated in the light source is detrimental to the operation and viewing of a liquid crystal display. The light source(s) discharge heat that is transferred to the image display panel, other electrical components in liquid crystal display, and the support structure of the liquid crystal display. Indeed, some of the electrical components in the display panel are themselves heat sources, which compounds the problem. However, these other components of the liquid crystal display normally possess poor thermal spreading properties and are not normally designed to dissipate heat away from the light source, especially in directions parallel to the image display panel face.
In addition, the illuminating light of a liquid crystal display remains in an energized state and at a consistent power level regardless of the image characteristics on the viewing panel. Variances in the image are control by the arrangement and alignment of the crystals in the image display panel. As such, the components of the liquid crystal display are in need of relief from the constant heat generated by the illuminating light. The constant heat generation can accelerate thermal deterioration of the liquid crystal material from which the display is formed and shorten the useful lifespan of the liquid crystal display device. Heat may also negatively affect the refresh rate of the screen.
The use of so-called “high orientation graphite film” as thermal interface materials for plasma display panels to fill the space between the back of the panel and a heat sinking unit is suggested by Morita, Ichiyanagi, Ikeda, Nishiki, Inoue, Komyoji and Kawashima in U.S. Pat. No. 5,831,374. However, the disclosure is centered on the use of pyrolytic graphite as the graphitic material and makes no mention of the use or distinct advantages of sheets of compressed particles of exfoliated graphite. In addition, the use of a heavy aluminum heat sinking unit is a critical part of the Morita et al. invention. In addition, U.S. Pat. No. 6,482,520 to Tzeng discloses the use of sheets of compressed particles of exfoliated graphite as heat spreaders (referred to in the patent as thermal interfaces) for a heat source such as an electronic component. Indeed, such materials are commercially available from Advanced Energy Technology Inc. of Lakewood, Ohio as its eGraf® SpreaderShield class of materials. The graphite heat spreaders of Tzeng are positioned between a heat generating electronic component and, advantageously, a heat sink, to increase the effective surface area of the heat generating component; the Tzeng patent does not address the specific thermal issues occasioned by display devices.
Graphites are made up of layer planes of hexagonal arrays or networks of carbon atoms. These layer planes of hexagonally arranged carbon atoms are substantially flat and are oriented or ordered so as to be substantially parallel and equidistant to one another. The substantially flat, parallel equidistant sheets or layers of carbon atoms, usually referred to as graphene layers or basal planes, are linked or bonded together and groups thereof are arranged in crystallites. Highly ordered graphites consist of crystallites of considerable size: the crystallites being highly aligned or oriented with respect to each other and having well ordered carbon layers. In other words, highly ordered graphites have a high degree of preferred crystallite orientation. It should be noted that graphites possess anisotropic structures and thus exhibit or possess many properties that are highly directional e.g. thermal and electrical conductivity and fluid diffusion.
Briefly, graphites may be characterized as laminated structures of carbon, that is, structures consisting of superposed layers or laminae of carbon atoms joined together by weak van der Waals forces. In considering the graphite structure, two axes or directions are usually noted, to wit, the “c” axis or direction and the “a” axes or directions. For simplicity, the “c” axis or direction may be considered as the direction perpendicular to the carbon layers. The “a” axes or directions may be considered as the directions parallel to the carbon layers or the directions perpendicular to the “c” direction. The graphites suitable for manufacturing flexible graphite sheets possess a very high degree of orientation.
As noted above, the bonding forces holding the parallel layers of carbon atoms together are only weak van der Waals forces. Natural graphites can be treated so that the spacing between the superposed carbon layers or laminae can be appreciably opened up so as to provide a marked expansion in the direction perpendicular to the layers, that is, in the “c” direction, and thus form an expanded or intumesced graphite structure in which the laminar character of the carbon layers is substantially retained.
Graphite flake which has been greatly expanded and more particularly expanded so as to have a final thickness or “c” direction dimension which is as much as about 80 or more times the original “c” direction dimension can be formed without the use of a binder into cohesive or integrated sheets of expanded graphite, e.g. webs, papers, strips, tapes, foils, mats or the like (typically referred to as “flexible graphite”). The formation of graphite particles which have been expanded to have a final thickness or “c” dimension which is as much as about 80 times or more the original “c” direction dimension into integrated flexible sheets by compression, without the use of any binding material, is believed to be possible due to the mechanical interlocking, or cohesion, which is achieved between the voluminously expanded graphite particles.
In addition to flexibility, the sheet material, as noted above, has also been found to possess a high degree of anisotropy with respect to thermal and electrical conductivity and fluid diffusion, comparable to the natural graphite starting material due to orientation of the expanded graphite particles and graphite layers substantially parallel to the opposed faces of the sheet resulting from very high compression, e.g. roll pressing. Sheet material thus produced has excellent flexibility, good strength and a very high degree of orientation.
Briefly, the process of producing flexible, binderless anisotropic graphite sheet material, e.g. web, paper, strip, tape, foil, mat, or the like, comprises compressing or compacting under a predetermined load and in the absence of a binder, expanded graphite particles which have a “c” direction dimension which is as much as about 80 or more times that of the original particles so as to form a substantially flat, flexible, integrated graphite sheet. The expanded graphite particles that generally are worm-like or vermiform in appearance, once compressed, will maintain the compression set and alignment with the opposed major surfaces of the sheet. The density and thickness of the sheet material can be varied by controlling the degree of compression. The density of the sheet material can be within the range of from about 0.04 g/cm3 to about 2.0 g/cm3. The flexible graphite sheet material exhibits an appreciable degree of anisotropy due to the alignment of graphite particles parallel to the major opposed, parallel surfaces of the sheet, with the degree of anisotropy increasing upon roll pressing of the sheet material to increase orientation. In roll pressed anisotropic sheet material, the thickness, i.e. the direction perpendicular to the opposed, parallel sheet surfaces comprises the “c” direction and the directions ranging along the length and width, i.e. along or parallel to the opposed, major surfaces comprises the “a” directions and the thermal, electrical and fluid diffusion properties of the sheet are very different, by orders of magnitude, for the “c” and “a” directions.
While the use of sheets of compressed particles of exfoliated graphite (i.e., flexible graphite) has been suggested as thermal spreaders, thermal interfaces and as component parts of heat sinks for dissipating the heat generated by a heat source (see, for instance, U.S. Pat. Nos. 6,245,400; 6,482,520; 6,503,626; and 6,538,892), the use of graphite materials has heretofore been independent, and not viewed as interrelated with other components, such as the frame system of display panels.
Conventional display devices typically utilize a thick, heavy metal support member (often a thick aluminum sheet, or set of multiple sheets) to which is attached both the display panel unit, the light source (which, in the case of LEDs, may be mounted to printed circuit boards, such as a metal core printed circuit board (PCB) with a thermally conductive dielectric material) and associated electronic components. Heat passing from these heat sources contributes to uneven temperature distributions created on the panel unit itself, which adversely affects the image presented on the display panels as well as display panel reliability.
The conventional support member provides both a mechanical function (i.e., for mounting the panel unit and associated electronics), as well as a thermal function (i.e., to help sink and spread heat generated by the light source(s) and/or the associated electronics). Accordingly, the support member is typically fabricated from a solid sheet of aluminum, on the order of about 2.0 mm thick. Expressed another way, the conventional display panel having a support member exhibits a support factor of about 440 mm-W/m° K or higher. The support factor is determined by multiplying the thickness of the support member present in the display panel by its in-plane thermal conductivity (thus, a 2.0 mm sheet of aluminum has a support factor of 440 mm-W/m° K, since the in-plane thermal conductivity of the high thermal conductivity aluminum typically employed is 220 W/m° K). It will be recognized that, since most metals are relatively thermally isotropic, the in-plane thermal conductivity is not substantially different from the through-plane thermal conductivity of the material.
A support member such as this can add a significant amount of weight, and can be expensive and difficult to construct, due to physical requirements, the need for many threaded mounting features for the electronics, and the high cost of high thermal conductivity aluminum sheet. Additionally, a framework (often made from steel or aluminum) is used to add further mechanical support to the support member, and allow for a robust mounting means for attachment of the display panel to a wall bracket or stand unit. Together, the framework and support member constitute a frame system in the conventional display panel.
LCD device manufacturers are under extreme pressure to reduce the cost and weight of their existing display solutions, while there has simultaneously been a desire to increase the brightness and luminous efficiency of the panel units. This can mean more power being sent to the light sources, which increases the thermal load on the system and requires additional heat dissipation capabilities within the display units. Active cooling solutions, such as fans and/or heat pipes, are undesirable due to unreliability, noise, and the fact that they contribute negatively to the cost and weight of the system. In addition to increasing brightness and luminous efficiency of the displays, display manufacturers are also under increasing pressure to produce larger panel sizes, which tends to increase the weight of the frame system (especially the support member) proportionately.
Thus, what is desired is a light weight and cost effective frame system for display devices, especially one which provides enhanced heat transfer capabilities, yet is structurally sound enough to provide both the attachment for the panel units and associated electronics, as well as the structural integrity for mounting and supporting the display device itself. The desired frame system reduces or eliminates the need for a support member, especially one formed of high conductivity aluminum.